In a large skillet, heat the 2 tablespoons of olive oil. Add the onion and cook over moderately low heat until softened, about 7 minutes. Add the Swiss chard and cook over moderate heat, stirring frequently, until wilted and no more liquid remains, about 8 minutes. Stir in the garlic and cook until fragrant, about 1 minute. Season the chard with salt and pepper. Transfer to a plate and let cool slightly.